1 Way Audio But Audio+video Is Fine

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Sporadically we get 1 way audio when one party is outside our firewall.

The caller is on NAT, and it works fine most of the time. Caller can hear the called party, same thing going the other direction. Caller can hear called party.

Asterisk 13.9 on Debian chan_sip with two identical Grandstream GXV3240 SIP phones

In the CLI, I can see calls on our PBX running media on port 5004 when the call is audio only. When the caller switches to video calls it opens ports
5004 and 5006 and everything works fine.

This does not make sense to me. It seems like audio on port 5004 should fail either way — adding another media channel for video should not affect audio.

I believe I have UDP ports 5000-40000 open right now on the firewall.

I also don’t understand why it varies from day to day.

Any ideas on how to debug or what might be happening?